In a variety of mechanical devices and systems, feed screw and nut arrangements are frequently used for providing translational motion to a moving carriage or the like. Typically, the nut is rigidly affixed to the carriage and is prevented from rotating about its axis, such that rotation of the feed screw causes the nut, and hence the carriage, to translate along the feed screw axis.
In some applications, precision of movement of the carriage is not of particular concern, and hence factors such as dimensional imperfections and friction which are common or inherent in feed screw/nut arrangements are relatively insignificant problems. However, in other applications requiring precise movement and positioning of a carriage, these factors can be significant problems. For example, small errors in the manufacture of the various components of a feed screw/nut arrangement can lead to various imperfections including misalignment of the feed screw axis with the direction of carriage movement, slight eccentricity of the nut with respect to the feed screw, and other problems. These dimensional imperfections can result in forces being exerted on the carriage in directions other than the intended direction of movement, which can cause deflection of the carriage or other undesirable consequences. Furthermore, in all feed screw/nut arrangements, there is friction between the rotating feed screw and the nut. In most conventional feed screw/nut arrangements wherein the nut is rigidly connected to the carriage, frictional forces exerted on the nut are transmitted to the carriage, which again can cause deflection of the carriage and other problems.
Because of problems such as those mentioned above, efforts have been made toward developing feed screw/nut arrangements having self-aligning nuts which can tolerate a certain amount of misalignment between the feed screw and the carriage or other structure on which the nut is carried. For example, U.S. Pat. No. 3,977,269 discloses a self-aligning nut mechanism having an elongate tubular nut body formed of a base portion and an internally threaded portion connected thereto. The internally threaded portion has a plurality of longitudinal slots that divide it into multiple elements whose thread formations are adapted for contact with the screw threads. The base portion of the body has a pair of oppositely disposed transverse slots enabling the threaded elements to have limited movement in radial directions with respect to the screw axis. Yieldable means are carried by the threaded elements for biasing them toward one another so as to maintain an intimate engagement of the threaded formations with the screw threads. An elongate spring sleeve surrounds and is concentric with the nut body, and has three pairs of oppositely disposed transverse slots spaced apart along the length of the sleeve. The three pairs of slots are indexed 90.degree. with respect to one another to enable pivotal movement of the adjacent sleeve parts. One extreme sleeve part carries the nut body and the other extreme sleeve part is adapted to be secured to a reciprocating part of a machine.
The self-aligning nut of the '269 patent thus purports to address the problem of misalignment between a feed screw and the reciprocating machine part, by allowing the nut to resiliently conform to the feed screw and maintain threaded engagement therebetween, and by allowing the sleeve parts to move relative to each other to compensate for slight misalignment between the screw axis and the machine part. However, the '269 patent does not purport to address the problem of forces being transmitted to the machine part in directions other than the intended direction of movement of the machine part. Even though the nut and sleeve arrangement purportedly compensates for misalignment and remains engaged with the feed screw, such misalignment would result in forces on the nut and/or on the machine part in directions other than the intended direction of movement, which forces are undesirable where highly precise movement of the machine part is required. Additionally, frictional forces between the feed screw and the nut would also result in such undesirable forces.